The Novikoff rat hepatoma cell line cannot use pentoses as a sole carbon and energy source when grown in culture. Variants of this cell line have been isolated which can use ribose, xylose and/or arabinose as a sole energy source. These variants are being examined to determine which of the relevant pentose catabolic enzymatic activities are being expressed. The influence of cell cycle, population density and carbon source on the levels of these enzymes will be examined. Mutants defective in specific pentose catabolic enzymes are being isolated by the BuDR-light method. Such mutants will serve as a source of altered proteins for further biochemical analyses of the pentose enzymes. Both pentose utilizing and non-pentose utilizing mutants are being used in cell hybridization experiments. Complementation analyses among pentose negative cell lines will be done to determine the minimum number of genes involved in the catabolic pathway. Interspecies hybridization will be used in an attempt to map the element(s) responsible for the original expression of the pentose utilizing phenotype. The use of microcells in intraspecies crosses will also be explored in an attempt to map the genes coding for the catabolic enzymes. Hybridizaton will also be used to obtain information on the regulation of the enzymes in liver cells.